Heterojunction bipolar transistor and method of manufacturing the same

ABSTRACT

A bipolar transistor is supported by a single-crystal silicon substrate including a collector contact region. A first epitaxial region forms a collector region of a first conductivity type on the collector contact region. A second epitaxial region forms a base region of a second conductivity type. Deposited semiconductor material forms an emitter region of the first conductivity type. The collector region, base region and emitter region are located within an opening formed in a stack of insulating layers that includes a sacrificial layer. The sacrificial layer is selectively removed to expose a side wall of the base region. Epitaxial growth from the exposed sidewall forms a base contact region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/783,469, now U.S. Pat. No. 10,244,423, filed Oct. 13, 2017, thedisclosure of which is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a heterojunction bipolar transistorand to a method of manufacturing a heterojunction bipolar transistor.

BACKGROUND

For high-frequency applications, bipolar transistors, and in particularheterojunction bipolar transistors (HBT), are currently used. It isknown to integrate bipolar transistors in standard CMOS methods.

The fabrication of a bipolar transistor from a stack of semiconductorregions forming the emitter, the base, and the collector of thetransistor poses various problems. In particular, a problem is to form abase contact region while keeping a low resistance of access to the baseregion and a low stray capacitance between the base and the collector.

United States Patent Application Publication No. 2017/0236923,incorporated by reference, teaches a heterojunction bipolar transistorand method of manufacturing. Concerns with this heterojunction bipolartransistor and method of manufacturing include: the process flow is toocomplicated; the resulting device suffers from concerns with robustness;the nitride remaining at the end of the process flow contributes toreliability issues; the process is difficult to implement at smallprocessing nodes (such as 28 nm) due to the height of the structureleading to difficulties with premetallization dielectric constructionand contact patterning modification; and the emitter resistance is notoptimal due to the emitter “plug” effect (a key parameter for high speedoperation).

It would thus be desirable to have a heterojunction bipolar transistorand a method of manufacturing a heterojunction bipolar transistor thatsolves at least some of the foregoing problems.

SUMMARY

In an embodiment, a method is provide for manufacturing a bipolartransistor in a structure including a single-crystal silicon substratecoated in succession with a first insulating layer, a silicon layer anda stack of layers comprising a sacrificial layer made of a firstmaterial arranged between two insulating layers made of a secondmaterial selectively etchable over the first material. The methodcomprises the steps of: a) etching an opening through the stack oflayers, the silicon layer and the first insulating layer to expose a topsurface of the substrate; b) laterally recessing the silicon layerwithin the opening to form an open region that annularly surrounds theopening; c) in the opening, forming by selective epitaxy from the topsurface of the substrate, to a level higher than a lower level of thestack, a collector region made of semiconductor material doped with afirst conductivity type, wherein the collector region closes off theannular open region to form an annular air spacer between the collectorregion and the silicon layer; d) in the opening, further forming byselective epitaxy from a top surface of the collector region, to a levelat least as high as an upper level of the sacrificial layer, a baseregion made of semiconductor material doped with a second conductivitytype; e) in the opening, further forming by deposition on a top surfaceof the base region, an emitter region made of semiconductor materialdoped with the first conductivity type; f) etching said stack to reachthe sacrificial layer; g) removing the sacrificial layer to expose aside wall of the base region; and h) forming a first portion of a basecontact region by epitaxy from the side wall of the base region exposedby the removal of the sacrificial layer.

In an embodiment, a bipolar transistor comprises: a single-crystalsemiconductor substrate; a first insulating layer over thesingle-crystal semiconductor substrate; a silicon layer over the firstinsulating layer; a stack of layers over the silicon layer; an openingextending through the stack of layers, the silicon layer and the firstinsulating layer; a recess of the silicon layer providing an open regionthat annularly surrounds the opening; a semiconductor collector regiondoped with a first conductivity type within the opening and resting onthe single-crystal semiconductor substrate, said semiconductor collectorregion closing off the annular open region to form an annular openspacer between the semiconductor collector region and the silicon layer;a semiconductor base region doped with a second conductivity type on topof the semiconductor collector region; a semiconductor emitter regiondoped with the first conductivity type on the semiconductor base regionand laterally extending beyond the base semiconductor region; and asingle crystal base contact region extending from a side wall of thesemiconductor base region and lying between two insulating layers ofsaid stack.

In an embodiment, a method comprises: forming a collector contact regiondoped with a first conductivity type in a semiconductor substrate;providing a first insulating layer over the collector contact region;providing a first silicon layer over the first insulating layer;depositing a stack of layers over the first silicon layer, said stack oflayers comprising a second insulating layer, a sacrificial layer and athird insulating layer; etching an opening extending through the stackof layers, the first silicon layer and the first insulating layer toexpose a portion of the semiconductor substrate at said collectorcontact region; laterally recessing the first silicon layer within theopening to form an open region that annularly surrounds the opening;epitaxially growing in said opening from the exposed portion of thesemiconductor substrate a collector region doped with the firstconductivity type that closes said open region to form an annular openspacer; epitaxially growing in said opening from the collector region abase region doped with a second conductivity type; depositing a secondsilicon layer doped with the first conductivity type in said opening onthe base semiconductor layer to form an emitter region; selectivelyremoving the sacrificial layer to expose a side wall of said baseregion; and epitaxially growing from the exposed side wall a firstportion of a base contact region.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with FIGS. 1 to 14, these drawings being cross-sectionviews illustrating steps of an embodiment of a method for manufacturinga heterojunction bipolar transistor.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the various drawings and, further, the various drawings are not toscale. For clarity, only those elements which are useful to theunderstanding of the described embodiments have been shown and aredetailed. In particular, certain masks used during the steps of themanufacturing method described hereafter have not been shown.

In the following description, terms “high”, “side”, “lateral”, “top”,“above”, “under”, “on”, “upper”, and “lower” refer to the orientation ofthe concerned elements in the corresponding drawings.

In FIG. 1, a structure comprises a single-crystal silicon substrate 101including insulating structures 103, for example, of the deep trenchisolation (DTI) type, are provided to delimit a location where aheterojunction bipolar transistor is desired to be formed. At thelocation of the transistor, substrate 101 comprises a heavily-dopedregion 105 of a first conductivity type, for example, type N. The region105 is flush with the upper (top) surface of substrate 101 and forms acollector contact region for the heterojunction bipolar transistor.

The upper surface of substrate 101 is coated with an insulating layer107 (that is made of silicon dioxide, for example) that is coated with asemiconductor material (for example, silicon) layer 109. Layer 109 maybe a polysilicon layer deposited on insulating layer 107. Layer 109 mayalso be a single-crystal silicon layer. As an example, the layer 109 maycorrespond to the silicon layer of a structure of SOI (Semiconductor OnInsulator) type. Layer 109 may have a doping of a second conductivitytype, for example, type P.

The deep trench isolation insulating structures 103 may be formed beforethe deposition of each of the layers forming the stack of layers 111,particularly in the case where silicon layer 109 is the silicon layer ofan SOI-type structure.

A stack of layers 111 rests on layer 109. The stack 111 comprises afirst insulating layer 119 that is coated with a second insulating layer113 (also referred to herein as a first sacrificial layer) that iscoated with a third insulating layer 115 that is coated with a fourthinsulating layer 117 (also referred to herein as a second sacrificiallayer). The material of layers 115 and 119 and the material of layers113 and 117 are selected to be selectively etchable over each other. Inan embodiment, the layers 113 and 117 are made of silicon nitride andthe layers 115 and 119 are made of silicon oxide. The stack 111 may, forexample, have a thickness of 80 nm (which is thinner than a comparablestack of United States Patent Application Publication No. 2017/0236923).

FIG. 2 shows the structure after the etching of an opening 121 thatcrosses completely through the stack 111, the silicon layer 109, and theinsulating layer 107 all the way to reach the collector contact region105 at the top surface of the substrate 101. The etch process used may,for example, comprise major anisotropic and minor isotropic etch steps.

FIG. 3 shows the structure after forming, in opening 21, a transistorcollector region 125 of the heterojunction bipolar transistor. Thecollector region 125 is made of a region of single-crystal siliconformed by selective epitaxy from the top surface of substrate 101. Thecollector region 125 has a thickness such that a top surface of thecollector region 125 is at least as high as a top surface of the layer109 and lower than or equal to a bottom surface of layer 115.Preferably, the level of the top surface of collector region 125 isslightly lower, for example, by from 1 to 3 nm, than the lower surfaceof layer 115. The collector region 125 is doped during the epitaxy or byimplantation after the epitaxy with the first conductivity type. As anexample, the collector region 125 is doped with phosphorus atoms,possibly associated with carbon atoms to limit the exodiffusion ofphosphorus atoms, and/or with arsenic atoms.

Because the lateral walls of opening 121 are not covered by aninsulating layer, there is a lateral etching of the silicon layer 109caused by the cycled epitaxial process, and this lateral etch forms openregions 126 at the side edge of the collector 125. The open regions 126are laterally closed off by the epitaxial growth of the collector region125 to form air spacers between the sidewalls of the collector 125 andthe semiconductor layer 109. The open regions 126 completely surroundthe collector region 125, and in this configuration will annularlysurround the opening 121. The annular region 126 is thus closed off bythe epitaxially grown collector region 125. The annular region 126 may,for example, have cross-sectional dimensions of a height in the range of10-30 nm and a width in the range of 10-50 nm.

Details of the cyclical epitaxy process used in the formation of thecollector region 125 are provided in U.S. application for patent Ser.No. 15/783,109, filed Oct. 13, 2017 entitled “Cyclic Epitaxy Process toForm Air Gap for Isolation for a Bipolar Transistor.”

FIG. 4 shows the structure after the forming, in opening 121, atransistor base region 127 comprising a stack of a doped region 127 aand a silicon capping region 127 b. The doped region 127 a of thetransistor base region 127 is made of a single-crystal semiconductormaterial such as silicon or, preferably, silicon-germanium as in thepresent embodiment. Doped region 127 a is formed by selective epitaxyfrom collector region 125. The doped region 127 a has a thickness suchthat a top surface of the doped region 127 a is higher than a topsurface of the insulating layer 113 but below the top surface of thelayer 115. The doped region 127 a is doped with the second conductivitytype, preferably during the epitaxy. As an example, doped region 127 ais doped with boron atoms, possibly associated with carbon atoms toavoid the exodiffusion of the boron atoms. The silicon capping region127 b is made of undoped single crystal semiconductor material and isformed by selective epitaxy from the top surface of the doped region 127a. The silicon capping region 127 b has a thickness such that a topsurface of the silicon capping region 127 b is below the top surface ofthe layer 115.

In FIG. 5, spacers 129 have been formed in the opening 121. The spacers129 rest on the top surface of silicon capping region 127 b and borderthe sidewall surfaces of at least the layer 115. An opening 123 isprovided between the spacers 129 to expose a top surface of the siliconcapping region 127 b. The spacers 129 have, for example, incross-sectional view, an L shape. As an example, the spacers 129 aremade of silicon oxide. The spacers are formed by forming a silicon oxidelayer on the sidewalls of the layer 115 and top surface of the siliconcapping region 127 b in the opening 121. A nitride deposit is then madeon the silicon oxide layer followed by an etch which preferentiallyremoves nitride material from horizontal surfaces. The nitride materialremaining after the etch forms a “D” shape spacer 131 which functions asa mask. This mask is used for perform a further etch to remove a portionof the silicon oxide layer to make the opening 123.

The remaining nitride material of the spacer 131 for the etch mask aswell as the silicon nitride layer (second sacrificial layer) 117 arethen removed by using an isotropic etching process. The result is shownin FIG. 6.

A silicon layer 133 doped with the first conductivity type is depositedall over the structure and fills openings 121 and 123. As an example,the silicon layer is deposited by RTCVD (“Reduced Temperature ChemicalVapor Deposition”), which enables the deposited silicon to bemonocrystalline at the interface with the silicon capping region 127 bof the transistor base region 127. A chemical mechanical polish may thenbe performed to planarize the top surface of the layer 133. An etch maskformed by a layer 135 (made of silicon oxide, for example) is thenformed on the planar top surface of layer 133. The result is shown inFIG. 7.

Convention lithographic processing is then performed to pattern the etchmask layer 135. An anisotropic etch is then performed to remove theunmasked portion of the doped silicon layer 133 and the unmasked portionof the oxide layer 115 all the way to nitride layer 113. The result isshown in FIG. 8. Thus, a portion 133′ of the silicon layer 133 is leftin place and forms the emitter region of the heterojunction bipolartransistor. The emitter region 133′ comprises a central portion restingon the transistor base region 127 at silicon capping region 127 b(extending through the opening 123) and a peripheral portion thatlaterally extends beyond the base region 127 and rests on a remainingportion of the layer 115.

A deposition of a silicon oxide layer 141 is made to cover the top oflayer 135 and the side walls of the emitter region 133′ and the sidewalls of the remaining portion of the oxide layer 115. Then, ananisotropic etch is performed to remove the silicon oxide from the topsurface of the first sacrificial layer 113. The result is shown in FIG.9. In an embodiment, the layer 135 may be removed prior to thedeposition of layer 141.

In the step illustrated by FIG. 10, the first sacrificial nitride layer113 has been removed by isotropic etching selective over the material oflayers 115, 119 and 141. This exposes the lateral side wall of the baseregion 127 (more specifically, a lateral side wall of the doped region127 a).

FIG. 11 shows the structure after formation of a first portion 149 of abase contact region 151. The first portion 149 is made of single-crystalsilicon formed by selective epitaxy from the lateral side wall of thebase region 127. Thus, the portion 149 of the base contact region 151 ismonocrystalline, which advantageously enables to decrease the resistanceat the interface between base contact region 151 and base region 127with respect to the case of a transistor which would have its basecontact region made of polysilicon. In this example, the first portion149 of base contact region 151 extends laterally all the way at least tothe peripheral edge of the remaining portion of layer 141.

Conventional isotropic etching techniques are then used to selectivelyremove a portion of insulating layer 119 and expose a top surface of thesemiconductor layer 109. The result is shown in FIG. 12.

FIG. 13 shows the structure after formation of a second portion 153 ofthe base contact region 151. The second portion 153 is made ofpolycrystalline silicon formed by selective epitaxy of doped silicon ofthe second conductivity type from the top surface of the polysiliconsemiconductor layer 109 and the lateral side wall of the first portion149. In the case where silicon layer 109 was not doped at the step ofFIG. 1, it may be doped by diffusion of dopant atoms from the basecontact region 151, and in particular from the first portion 149.

An example of the configuration for making electrical contact to theemitter (E), base (B) and collector (C) terminals of the heterojunctionbipolar transistor is shown in FIG. 14. A premetallization dielectriclayer 161 covers the structure. A layer of silicide 163 is provided ateach contact location with the collector contact region 105, the emitterregion 133′ and the base contact region 151. A metal contact plug 165extends through the premetallization dielectric layer 161 to makecontact with the silicide 163.

Advantageously, if the starting point for the substrate is a SOI-typestructure, then the semiconductor layer 109 is made of single-crystalsilicon and the second portion 153 of base contact region 151 will alsobe made of single-crystal silicon grown by selective epitaxy.Advantageously, the entire base contact region 151 in such animplementation will be made of single-crystal silicon which enables todecrease the resistance of base contact region 151, and thus decreasethe resistance of access to the base region 127 with respect to the caseof a transistor where all or part of the base contact region is made ofpolysilicon.

Access to the collector contact region 105 for the purpose of exposingthe top surface of the substrate 101 can be achieved by forming a maskover the emitter region 133′ and on portions of the base contact region151 that laterally extend adjacent to the emitter region 133′. An etchmay then be performed through the mask. The premetallization dielectriclayer 161 may then be deposited over the structures.

The silicide process to form silicide layers 163 may be performed usingwell known techniques at any suitable point in the fabrication process.The silicide layers 163 are formed at the upper surface of the emitterregion 133′, on the upper surface of collector contact region 105, andon the upper surface of the base contact region 151.

In an alternative implementation, the first and second portions 149 and153 of the base contact region 151 may be formed simultaneously. Toachieve this, the nitride layer 113 is removed to expose the lateralside wall of the base region 127 and the portion of insulating layer 119is removed to expose the top surface of the semiconductor layer 109.These removal steps are performed prior to performing any epitaxialgrowth. Then, the first and second portions 149 and 153 aresimultaneously formed by epitaxy from the side wall of base region 127and the top surface of silicon layer 109. A structure similar to thatshown in FIG. 12 is then obtained.

In the transistor of FIG. 14, the base contact region 151 is insulatedfrom the collector region 125 by the air spacer 126 and the remainingportion of the layer 119. The base contact region 151 makes contact withthe base region 127 at the level of at least part of the side wall ofthe base region 127. This structure minimizes the risk of diffusion ofdopant atoms from the base contact region 151 to the collector region125. As a result, there is a reduction in base-collector capacitance.Still further, this permits an increase in the doping level of the basecontact region 151 so as to achieve a decrease in access resistance tothe base region 127 without concern that this increased doping will leadto an increase in the base-collector capacitance.

Advantageously, in the transistor of FIG. 14, the shortest conductivepath between the base region 127 and silicide 163 for the base contactregion 151 is made of single-crystal semiconductor material. As aresult, the resistance of access to the base region 127 is reduced.

Further, in the transistor of FIG. 14, the separation between the upperlevel of base contact region 151 and the lower level of emitter region133′ can be controlled by the process step of epitaxial growth of thesecond portion 153 so as to decrease the stray capacitance between basecontact region 151 and the emitter region 133′.

Advantageously, the previously-described method enables to form inself-aligned fashion the collector region 125, base region 127, andemitter region 133′ of a bipolar transistor without providing manymasking and/or etch steps.

Each step of the previously-described method is a step currently used instandard CMOS methods, whereby this method is compatible with standardCMOS methods.

As an example, the various previously-described layers, regions,portions may have the following dimensions:

a thickness in the range from 10 to 75 nm, for example, 25 nm, forinsulating layer 107;

a thickness in the range from 3 to 20 nm, for example, 7 nm, for siliconlayer 109;

a thickness in the range from 10 to 40 nm, for example, 20 nm, forinsulating layer 113;

a thickness in the range from 5 to 20 nm, for example, 10 nm, forinsulating layer 119;

a thickness in the range from 10 to 50 nm, for example, 25 nm, forlayers 115 and 117;

a width from 0.1 to 0.3 μm, for example, 0.2 μm, for opening 121; and

a thickness in the range from 50 to 200 nm, for example, 75 nm, for thesilicon layer 133.

The doping levels of the various previously-described layers, regions,portions will be selected conventionally.

Specific embodiments have been described. Various alterations,modifications, and improvements will occur to those skilled in the art.In particular, the number and the order of the steps of thepreviously-described method may be adapted by those skilled in the art.For example, the steps of cleaning the exposed surfaces of thesemiconductor regions from which the epitaxies are performed may beprovided before each epitaxy step. Additional spacer structures may beprovided as needed. During the step of forming the collector region 125,only a central and/or lower portion of region 125 may be doped byselective implantation of dopant atoms.

It will readily occur to those skilled in the art that thepreviously-indicated conductivity types for the layers, regions, etc.may all be inverted.

Although an embodiment of a method where the base region 127 is made ofsilicon-germanium has been described, the base region 127 may also beformed by epitaxy of silicon, germanium, or another semiconductormaterial capable of growing by epitaxy from silicon and from whichsilicon can grow by epitaxy. For example, this method may be used tomanufacture transistors using III-V semiconductors.

Alterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and the scopeof the present invention. Accordingly, the foregoing description is byway of example only and is not intended to be limiting. The presentinvention is limited only as defined in the following claims and theequivalents thereto.

The invention claimed is:
 1. A bipolar transistor, comprising: asingle-crystal semiconductor substrate; a first insulating layer overthe single-crystal semiconductor substrate; a silicon layer over thefirst insulating layer; a stack of layers over the silicon layer; anopening extending through the stack of layers, the silicon layer and thefirst insulating layer; a recess of the silicon layer providing an openregion that annularly surrounds the opening; a semiconductor collectorregion doped with a first conductivity type within the opening andresting on the single-crystal semiconductor substrate, saidsemiconductor collector region closing off the annular open region toform an annular open spacer between the semiconductor collector regionand the silicon layer; a semiconductor base region doped with a secondconductivity type on top of the semiconductor collector region; asemiconductor emitter region doped with the first conductivity type onthe semiconductor base region and laterally extending beyond the basesemiconductor region; and a single crystal base contact region extendingfrom a side wall of the semiconductor base region and lying between twoinsulating layers of said stack.
 2. The transistor of claim 1, whereinsaid single crystal base contact region extends between the twoinsulating layers of said stack.
 3. The transistor of claim 1, whereinsaid single crystal base contact region includes a first single crystalportion extending from the side wall of the semiconductor base regionand lying between two insulating layers and a second single crystalportion extending from the silicon layer.
 4. The transistor of claim 1,further comprising: a silicide layer a top surface portion of each ofthe single-crystal silicon substrate, the single crystal base contactregion and the emitter region; a premetallization dielectric layer; andmetal contacts extending through the premetallization dielectric layerto reach each silicide layer.
 5. The transistor of claim 1, wherein thesingle-crystal silicon substrate, first insulating layer and siliconlayer form a silicon on insulator (SOI) type substrate.
 6. Thetransistor of claim 1, wherein the silicon layer is a single-crystalsilicon layer.
 7. The transistor of claim 6, wherein the silicon layeris doped with the second conductivity type.
 8. The transistor of claim1, further comprising a collector contact region located within thesingle-crystal silicon substrate under the semiconductor collectorregion doped with the first conductivity type.
 9. The transistor ofclaim 1, wherein said two insulating layers are made of silicon nitride.10. The transistor of claim 1, wherein the semiconductor collectorregion is made of a silicon material and wherein the semiconductor baseregion is made of a silicon-germanium material and wherein thesemiconductor emitter region is made of a silicon material.
 11. Thetransistor of claim 1, wherein the semiconductor base region includes afirst portion doped with the second conductivity type and a secondportion that is undoped.
 12. The transistor of claim 1, wherein theannular open spacer has cross sectional dimensions of a height in arange of 10-30 nm and a width in a range of 10-50 nm.
 13. A bipolartransistor, comprising: a semiconductor substrate; a first insulatinglayer over the semiconductor substrate; a silicon layer over the firstinsulating layer; a second insulating layer over the silicon layer anopening extending through the second insulating layer, the silicon layerand the first insulating layer; a recess of the silicon layer providingan open region that annularly surrounds the opening; a semiconductorcollector region doped with a first conductivity type within the openingand resting on the semiconductor substrate, said semiconductor collectorregion closing off the annular open region to form an annular openspacer between the semiconductor collector region and the silicon layer;a semiconductor base region doped with a second conductivity type on topof the semiconductor collector region; a semiconductor emitter regiondoped with the first conductivity type on the semiconductor base regionand laterally extending beyond the base semiconductor region; and a basecontact region extending from a side wall of the semiconductor baseregion and lying on said second insulating layer.
 14. The transistor ofclaim 13, further comprising a third insulating layer over the basecontact region, wherein the opening extends through the secondinsulating layer and wherein the third insulating layer is adjacent theside wall of the semiconductor base region is located within theopening.
 15. The transistor of claim 14, wherein said base contactregion includes a first portion extending from the side wall of thesemiconductor base region and lying between the second and thirdinsulating layers and a second portion extending from the silicon layer.16. The transistor of claim 14, wherein said second and third insulatinglayers are made of silicon nitride.
 17. The transistor of claim 13,wherein the silicon layer is in contact with the base contact regionthrough an epitaxial extension.
 18. The transistor of claim 13, furthercomprising: a silicide layer a top surface portion of each of thesilicon substrate, the base contact region and the emitter region; apremetallization dielectric layer; and metal contacts extending throughthe premetallization dielectric layer to reach each silicide layer. 19.The transistor of claim 13, wherein the silicon substrate, firstinsulating layer and silicon layer form a silicon on insulator (SOI)type substrate.
 20. The transistor of claim 13, wherein the siliconlayer is a single-crystal silicon layer and wherein the silicon layer isdoped with the second conductivity type.
 21. The transistor of claim 13,further comprising a collector contact region located within the siliconsubstrate under the semiconductor collector region doped with the firstconductivity type.
 22. The transistor of claim 13, wherein thesemiconductor collector region is made of a silicon material and whereinthe semiconductor base region is made of a silicon-germanium materialand wherein the semiconductor emitter region is made of a siliconmaterial.
 23. The transistor of claim 13, wherein the semiconductor baseregion includes a first portion doped with the second conductivity typeand a second portion that is undoped.
 24. The transistor of claim 13,wherein the annular open spacer has cross sectional dimensions of aheight in a range of 10-30 nm and a width in a range of 10-50 nm.